1. Field of the Invention
This invention relates generally to pre-load mechanisms, and more particularly to a pre-load mechanism having a self-mounting coil spring that resiliently forces a pre-loading member against a pre-loaded member. The inventive principles are especially advantageous in electric-operated fuel injectors of the type used in internal combustion engines that power automotive vehicles, although it is believed that inventive principles may be utilized to advantage in various valves and in detent mechanisms.
2. Background Information
Each of U.S. Pat. Nos. 5,494,224 and 5,494,225 describes an electric-operated fuel injector for an internal combustion engine. The fuel injector has an armature/needle valve assembly that, in closed condition of the fuel injector, is resiliently biased by a helical coil pre-load spring to cause a tip end of the needle valve to forcefully seat against a valve seat member in closure of a through-hole at the center of the valve seat member. The fuel injector also has an electromagnet coil that, when energized by an electric control circuit to which it is connected, operates the fuel injector to open condition. The electromagnetic force acts on the armature to overcome the pre-load force and unseat the tip end of the needle valve from the valve seat member, thereby opening the through-hole in the valve seat member so that pressurized liquid fuel can be injected into the engine from a nozzle end of the fuel injector.
Calibration of the fuel injector comprises setting axial compression of the pre-load spring in an amount that results in the exertion of a certain axial pre-load force on the valve seat member by the seated tip of the needle valve when the electromagnet coil is not being energized.
The fuel injector further comprises a fuel inlet tube through which fuel passes axially from an inlet end into the interior of the fuel injector. One axial end of the pre-load spring is disposed within a lower axial end of the fuel inlet tube; the opposite axial end of the spring is received within a counterbore in one axial end of the armature, bearing against an internal shoulder at an internal end of the counterbore. In a calibrated fuel injector, the one axial end of the spring that is disposed within the lower end of the fuel inlet tube bears axially against an axial end of an adjustment tube which is telescopically received within, and joined with, the fuel inlet tube, and which serves to convey fuel along that portion of the length of the fuel inlet tube with which it is axially coextensive.
The calibration process comprises axially positioning the adjustment tube relative to the fuel inlet tube, before the two tubes are joined together, to set the axial compression of the pre-load spring in an amount that results in the exertion of a certain axial pre-load force on the valve seat member by the seated tip of the needle valve. Then the two tubes are joined, such as by mechanically staking, or crimping, one to the other.
Calibration may, for example, be performed at a station of an automated fuel injector processing line where a fuel injector is operated to open condition and liquid is flowed through it. A servo-motor-controlled actuator at the station positions the adjustment tube axially within the fuel inlet tube, compressing the pre-load spring in the process, as liquid flow through the open fuel injector is measured. When a desired flow is achieved, the two tubes are axially locked together by crimping the outside diameter (O.D.) of the fuel inlet tube such that its inside diameter (I.D.) deforms sufficiently onto the O.D. of the adjustment tube to prevent relative axial movement between the two tubes without impeding flow through the adjustment tube.
It is believed that this crimping step may at times cause an unintended component of relative axial movement between the two tubes that affects the intended calibration. Accordingly, replacement of this crimping step by a different step that would eliminate the possibility of accompanying, unintended, relative axial movement between a fuel inlet tube and an adjustment tube is seen to be desirable.
It has heretofore been proposed to employ a roll pin in place of an adjustment tube. The roll pin is increasingly pressed, with a press-fit, into the fuel inlet tube by a mandrel until the intended calibration is attained. While this process eliminates the crimping step, it is not reversible. Hence, it is believed that extensive care must be taken in order to avoid the occurrence of irreversible axial overtravel when pressing a roll pin into a fuel inlet tube. Lack of adequate control may produce excessive axial overtravel of a roll pin that results in scrapping of in-process parts.